In a nuclear reactor, such as a boiling water reactor (BWR), a plurality of elongate fuel rods are laterally spaced apart to form a fuel bundle. The fuel rods are typically cylindrical tubes, each containing nuclear fuel pellets stacked therein, with each tube having solid cylindrical end plugs, or endcaps, fixedly joined, by welding for example, to the ends of the tube to form a closed component.
The several fuel rods are typically configured in a square array, such as an 8 by 8 square array, with the individual fuel rods being spaced from each other by upper and lower tie plates at opposite ends thereof, with a plurality of spacer grids longitudinally spaced apart between the tie plates. The tie plates and grids have individual cells which receive respective ones of the fuel rods for maintaining the lateral spacings between adjacent fuel rods.
Each of the fuel rods is fabricated by cutting a suitable length of tubing to form two ends, with each tube then being conventionally filled with the nuclear fuel pellets. The tube endcaps, which have the same outer diameter as that of the tube, are then joined to the respective ends of the tube and welded thereto. In order to properly align the endcaps with the tube itself, both mating surfaces of the tube end and the endcap must be substantially "square" with the respective mating surfaces being disposed substantially perpendicularly to the longitudinal centerline axis of the tube. If an endcap is not squarely mounted to the tube end for being coextensive therewith, it will be tilted or skewed relative to the tube centerline axis with a portion of the endcap projecting radially outwardly from the outer diameter of the tube itself. The resulting misalignment of the tube endcap and the tube itself affects the accuracy with which the completed tube may be assembled into the fuel bundle, with excessive misalignment of the endcap preventing proper assembly thereof.
Accordingly, during the manufacturing process, squareness of each of the fuel rod ends is conventionally measured to determine whether it is within acceptable tolerances, and if not, the tube end must be remachined to meet required squareness tolerances. One conventional indicator of tube end squareness is called total indicator runout (TIR) which is obtained by placing a conventional dial indicator adjacent to the tube end, with the dial indicator stylus disposed in contact with the tube end generally parallel to the longitudinal centerline axis of the tube. The tube is then rotated about its centerline axis in a conventional jig, with the dial indicator being read to determine the maximum difference in axial position of the tube end between minimum and maximum points thereon, which difference is the TIR reading.
Since each fuel rod is about four meters long and has an outer diameter of about 12 mm, rotating the entire fuel rod about its longitudinal centerline axis requires specialized equipment to prevent damaging the tube itself and forming undesirable scratches in the outer surface thereof. Furthermore, the stylus of the dial indicator necessarily rubs along the tube end as it is rotated which leads to wear of the stylus, and may also possibly damage the rod end.